1. Field of the Invention
The present invention relates generally to improved capacity of defective materials and more specifically to the preparation of defective metal oxides for battery cathodes with increased specific energy and improved lithium capacity.
2. Discussion of the Background
The revolution in the portable electronics industry has increased the demand for lightweight, high-energy batteries. Strategies for improving the energy density of batteries include technical challenges such as (1) increasing the voltage difference between the cathode and anode, (2) decreasing the weight of the materials, and (3) increasing the charge-storage capacity of the materials.
The obstacles to these improvements are often physical. For instance, the lithium (Li) capacity of metal-oxide charge-storage materials is limited because the valence of the metal cations fixes the number of electrons withdrawn from each metal center, as shown for the V2O5-cathode half-cell reaction in Eq. 1. Only one electron is consumed when a lithium ion from an electrolyte solution (Li(sol)+) inserts into a V2O5 cathode and a V5+ ion is reduced to V4+ (Eq. 1).Vv+Li(sol)++e−=Vv′+Lii•  (1)Using Kröger-Vink notation, Vv designates a V5+ ion at a vanadium-cation site in the V2O5 lattice, and Vv′ represents the occupation of the cation site with a V4+ ion, leaving it effectively 1-negative (′). Lii• represents a lithium ion that is located in an interstitial site (i) and has an effective 1-positive charge (•). The Li+ may actually be associated with an oxygen anion, but this defect is electrically and site equivalent to a lithium interstitial. Kröger-Vink notation, which is used to write equilibrium reactions and mass action equations in defective oxides, is also useful for writing equations for the metal oxides used in batteries, because it demands site and charge balance in addition to chemical balance and includes defects (vacancies, etc.) as chemical species.
The vanadium cations in bulk V2O5 are reduced to an average oxidation state of +4.5 with Li+ insertion, because structural constraints allow reduction of only half of the V5+ ions to V4+. This physical limitation of 0.5 electron stored per vanadium ion is broken when Li+ is inserted into V2O5 materials synthesized by sol-gel methods. Up to 2.5 Li+ can be inserted per vanadium ion into the amorphous, high surface area, high porosity frameworks of V2O5 aerogels and xerogels, resulting in capacities as high as 600 mAh/g. Although Eq. (1) predicts V3+ defects should be formed when 3 to 5 equivalents of Li+ are inserted into V2O5 xerogels and aerogels, X-ray absorption spectroscopy (XAS) studies indicate that only V4+ and V5+ ions are present in the fully discharged materials. Raman spectroscopy shows unique vibrational bands in Li—V2O5 xerogels, but provides no identification of the mechanism for the additional Li+ insertion. Even if the metal-oxide materials become metallic, the charge balance constraints in Eq. 1 still apply.